1. Technical Field
The present invention relates to a semi-finished lens of plastic material which will be later completed by grinding on the basis of a spectacles prescription.
2. Technical Background
Plastic lenses having a usual range of power degrees are produced by the cast-molding process, and prescribed lenses produced on the basis of spectacles prescriptions, particularly those lenses which are powerful and which have both spherical power and astigmatic power, are produced mostly from prefabricated semi-finished lenses by grinding on the basis of spectacles prescriptions.
As for grinding operation of such semi-finished lenses, grinding both surfaces on the basis of a spectacles prescription is less efficient than preparing one surface as a molded lens surface which needs no grinding and the other surface as a nonprocessed surface and grinding the latter on the basis of a prescription.
In the prior art, as shown in FIG. 3, a semi-finished lens L1' of plastic material is produced, with its concave surface 11 molded using a mold having a first surface of a given curvature so that the lens L1' has a spherical nonprocessed surface b' corresponding to the first surface of the mold, and with its convex surface 12 molded using a mold having a second surface of a given curvature and ground with high precision so that the lens L1' has a spherical molded surface a' which corresponds to the second surface of the mold and which needs no grinding. Thus, when a spectacles prescription is determined for a customer, the nonprocessed concave surface 11 is ground on the basis of said prescription and, the prescribed lens L2' desired by the customer as shown in FIG. 4 is thus provided.
However, to finish said conventional type of semi-finished lens to provide a prescribed lens, the concave surface 11 has to be ground. Further, this concave surface is spherical; therefore, if a lens having a complicated curved surface, for example, a toric lens for astigmatism is to be ground, a variety of complicated jigs are required and so this is a very complicated technique. Therefore, there have been drawbacks that a lens processed with high precision cannot be obtained and that too much time is required for processing.
Among single-focus lenses of plastic material, prescribed lenses completed by grinding semi-finished lenses are chiefly those having special degrees of power outside the range of those for lenses usually produced by lens makers and include considerably powerful lenses having high degrees of spherical power and astigmatic power. Therefore, a lens completed by grinding the concave surface 11, as described above, has a relatively large edge thickness B, a fact which is not desirable from the standpoint of use and appearance.
Further, in conventional toric lenses having spherical power and astigmatic power, since one surface is a toric surface and the other is a spherical or flat surface, lenses produced by grinding have the drawback of having spherical aberration, and since they are powerful as described above, this problem of spherical aberration becomes more serious.